ISTANBUL—Turkey’s government submitted a bill Saturday to overhaul the constitution and concentrate power in President
Recep Tayyip Erdogan’s
office, a controversial move that risks further destabilizing the country amid myriad security and economic threats.

The bill includes measures that would call for Turkey’s president to run his own cabinet and acquire executive powers currently vested in the parliamentary government. The changes give the head of state sweeping powers to govern by decree, and could potentially allow Mr. Erdogan to rule for two, five-year terms until 2029, the draft legislation shows.

The passage of the bill would send these amendments to a national referendum that would likely be held in late spring or summer. The changes pertaining to Turkey’s governance, if accepted, would go into effect in 2019.

“God willing, this will be the start of a new era,” Mr. Erdogan said during a rally in Istanbul shortly before his party introduced the bill in Ankara. “Are you ready for all of this,” the president asked a crowd of flag waving supporters, who responded with a resounding “yes.”

The 21-article legislation follows two months of secretive negotiations during a state of emergency between the ruling Justice and Development Party—known as AKP and founded by Mr. Erdogan—and the opposition Nationalist Movement Party, or MHP.

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Some of the items relate to the judiciary, such as abolishing military tribunals, reducing Constitutional Court justices by two to 15 and boosting the president and parliament’s power to elect the members of Turkey’s top judiciary body.

Under the proposed changes the number of lawmakers in parliament would increase to 600 from 550, while the age limit to run for office drops to 18 from 25. There would also be substitute lawmakers to replace those who leave parliament. Turkey would hold local elections in March 2019, followed by simultaneous presidential and parliamentary elections in November.

The talks on empowering the presidency with executive powers haven’t included Turkey’s largest opposition party, amid criticism from its leadership that the proposed changes would undermine democracy. Pro-Kurdish lawmakers, who command a bigger bloc than nationalists in parliament, have also been sidelined and authorities jailed their leadership in November on terrorism charges—moves they have criticized as a political witch hunt.

Together the AKP and MHP have enough seats in parliament to pass the bill. But victory isn't a foregone conclusion: it is unclear whether these parties command total party discipline—and public support for the radical changes is also uncertain.

“The proposed presidential system would alleviate the uncertainty in Turkey’s political system in the short run,” said
Ozgur Unluhisarcikli,
Turkey director at the German Marshal Fund of the U.S. “It could also lead to more instability in the future: failure of the referendum would bring into question the governing style of President Erdogan while a positive outcome could lead to the marginalization of the parliament and increase political tension.”

A crowd listens to Turkish President Recep Tayyip Erdogan at an event in Istanbul on Saturday.
Photo:
Tolga Adanali/Zuma Press

Since the failed coup, the president has ruled by decree under emergency powers and overseen a sweeping purge. More than 125,000 soldiers, journalists, teachers and bureaucrats have been suspended or dismissed—with some 36,000 detained—for suspected links terrorist organizations. The government also has seized companies with combined assets worth more than $10 billion, for suspected links to coup-plotters.

Mr. Erdogan has long advocated for a presidential system similar to the U.S. and Russia, and argues that Turkey can fend off existential threats only with a strong and efficient state.

After becoming Turkey’s first directly elected president in 2014 after three terms as prime minister, Mr. Erdogan has repeatedly said his popular mandate has already effectively transformed the presidency, imbuing it with de facto executive powers.

“This is a dictatorship project, an undertaking to monopolize all of the state’s powers,” said
Bulent Tezcan,
deputy chairman of the main-opposition Republican People’s Party, or CHP.

Mr. Erdogan has dismissed his critics, saying he has acted in line with the national will. He argues that the amendments will fix constitutional ambiguities that could trigger a leadership crisis if the prime minister and the president don’t work in harmony. Mr. Erdogan has already ousted his first handpicked premier and replaced him with a loyalist.

The ruling AKP is 14 lawmakers short of the 330 needed to adopt the bill and put the reforms to a referendum.

The AKP is banking on the 39 seats held by MHP, and ruling party officials and some nationalists say the proposal will pass easily. Other lawmakers believe that there could be as many as 20 defections from AKP and MHP. All ruling party lawmakers signed the proposal to overhaul the constitution, in uniform support for the changes.

One of five MHP lawmakers who already pledged to vote against the bill,
Nuri Okutan
said he believes others will follow him, given the longstanding internal revolt against MHP Chairman Devlet Bahceli. He slammed the party chief for aiding Mr. Erdogan’s push for a “fait accompli.”

The MHP didn’t respond to a request for comment.

Should the amendments clear parliament, most observers say Mr. Erdogan, whose popularity rating in local surveys ranges from 50% to 55%, would rally enough yes votes to secure a referendum victory.

In a late-November survey by Turkish pollster A&G, 45.7% of the participants said they supported a presidential system, while 41.6% opposed it and 12.7% were undecided. The margin of error wasn’t publicly available, but poll results showed the yes vote rising from 37.4% in June as the share of undecided dropped from 20.1% and the no vote declined from 42.5%.

Failure to change the constitution would likely lead to early elections, officials said.

Many lawmakers, government officials and outside observers say Turkey’s fractious parliamentary system also faces deep overhauls should the reforms pass. They predict an evolutionary shift to a two-party legislative branch similar to that in the U.S., and the slow death of minority parties.

That would effectively destroy the MHP and pro-Kurdish Peoples’ Democratic Party. The two parties’ gains in July 2015 polls cost Mr. Erdogan’s party its ruling majority, prompting the president to call snap elections that restored the AKP’s power.

The shift would also spell the end of coalition governments, which Mr. Erdogan said have been responsible for bouts of instability and economic malaise. The AKP swept to power in 2002 after a decade of coalition governments that marked the worst period of Turkey’s Kurdish insurgency and ended in a financial crisis.

“We have started a process that would always produce a powerful political authority, that would bring stability,” said Prime Minister
Binali Yildirim,
whose office would become obsolete if the measure is adopted. “Let it be auspicious.”